The more than 1 million-square-foot office complex was built in the late 1980s and 1990s.

Fortis Property CEO Jonathan Landau said Wednesday that his firm plans to start remodeling the project next month.

"We are going to make some great improvements to the buildings," Landau said.

Most of the redo will concentrate on the atrium that connects the three office towers. But Fortis also plans to make changes in the lobby areas of the buildings and upgrade the elevators.

"We have a great new design for the atrium that will create a more pleasant atmosphere for our tenants," Landau said. "The challenge when you take a 1980s building and try to modernize it is to make it look cohesive.

"We are going to try and make it a very uniform feel."

Dallas design firm Staffelbach is in charge of the makeover.

The work should be done by the end of the year.

The buildings are more than 90 percent leased.

"We've done a bunch of new leasing and tenant renewals," Landau said.

Fortis' purchase of the Colonnade from CBRE Global Investors is one of the largest office investments in the Dallas area so far in 2013. The project sold for near $200 million.